Spatial dynamics of overbank sedimentation in floodplain systems

Floodplains provide valuable social and ecological functions, and understanding the rates and patterns of overbank sedimentation is critical for river basin management and rehabilitation. Channelization of alluvial systems throughout the world has altered hydrological and sedimentation processes within floodplain ecosystems. In the loess belt region of the Lower Mississippi Alluvial Valley of the United States, channelization, the geology of the region, and past land-use practices have resulted in the formation of dozens of valley plugs in stream channels and the formation of shoals at the confluence of stream systems. Valley plugs completely block stream channels with sediment and debris and can result in greater deposition rates on floodplain surfaces. Presently, however, information is lacking on the rates and variability of overbank sedimentation associated with valley plugs and shoals.We quantified deposition rates and textures in floodplains along channelized streams that contained valley plugs and shoals, in addition to floodplains occurring along an unchannelized stream, to improve our understanding of overbank sedimentation associated with channelized streams. Feldspar clay marker horizons and marker poles were used to measure floodplain deposition from 2002 to 2005 and data were analyzed with geospatial statistics to determine the spatial dynamics of sedimentation within the floodplains.Mean sediment deposition rates ranged from 0.09 to 0.67 cm/y at unchannelized sites, 0.16 to 2.27 cm/y at shoal sites, and 3.44 to 6.20 cm/y at valley plug sites. Valley plug sites had greater rates of deposition, and the deposited sediments contained more coarse sand material than either shoal or unchannelized sites. A total of 59 of 183 valley plug study plots had mean deposition rates > 5 cm/y. The geospatial analyses showed that the spatial dynamics of sedimentation can be influenced by the formation of valley plugs and shoals on channelized streams; however, responses can vary. Restoration efforts in the region need to have basinwide collaboration with landowners and address catchment-scale processes, including the geomorphic instability of the region, to be successful.     

Role of hydrological regime and floodplain sediments in channel instability of the Bhagirathi River,Ganga-Brahmaputra Delta,India

Abstract

This research deals with the surface dynamics and key factors – hydrological regime, sediment load, and erodibility of floodplain facies – of frequent channel shifting, intensive meandering, and lateral instability of the Bhagirathi River in the western part of the Ganga-Brahmaputra Delta (GBD). At present, the floodplain of the Bhagirathi is categorized as a medium energy (specific stream power of 10–300 W m^?2), non-cohesive floodplain, which exhibits a mixed-load and a meandering channel, an entrenchment ratio >2.2, width–depth ratio >12, sinuosity >1.4, and channel slope <0.02. In the study area, since 1975, four meander cutoffs have been shaped at an average rate of one in every 9–10 years. In the active meander belt and sand-silt dominated floodplains of GBD, frequent shifting of the channel and meander migration escalate severe bank erosion (e.g. 2.5 × 10⁶ m³ of land lost between 1999 and 2004) throughout the year. Remote sensing based spatio-temporal analysis and stratigraphic analysis reveal that the impact of the Farakka barrage, completed in 1975, is not the sole factor of downstream channel oscillation; rather, hydrogeomorphic instability induced by the Ajay–Mayurakshi fluvial system and the erodibility of floodplain sediments control the channel dynamics of the study area.     

STORAGE OF MINING-RELATED ZINC IN FLOODPLAIN SEDIMENTS,BLUE RIVER,WISCONSIN

This study investigates spatial patterns of sediment-associated zinc (Zn) storage in floodplain deposits and the potential reintroduction of these metal-contaminated sediments to the active channel by bank erosion. We estimate patterns of Zn mass storage by combining longitudinal trends in Zn concentrations with measurements of sediment mass storage in overbank and point-bar deposits. Overbank deposits are the largest contaminant sink, storing five times more Zn than the point-bar deposits. While Zn concentrations decrease downstream because of dilution effects, the total mass of Zn stored in floodplains is greatest in both the upstream reaches and in the wider lower valleys where low channel gradients promote rapid sedimentation. Zn storage is low in middle reaches where steep, narrow valleys with high stream power favor sediment transport over deposition. Overall, more than half of the Zn released by mining remains stored in floodplain deposits within the watershed. The remobilization of Zn from storage is more likely in the upstream and mid-basin reaches where high stream power increases rates of lateral channel migration. Channels in the lower valley lack the stream power to migrate laterally and remobilize the large Zn mass stored in overbank sediments. [Key words: floodplains, Zn contamination, mining, stream power, Wisconsin.]     

Floodplain development based on selective preservation of sediments, Squamish River, British Columbia

In the conventional model of floodplain sediment accumulation, mechanisms of floodplain growth are differentiated into lateral and vertical accretion processes, in which within-channel deposits are capped by overbank deposits. In the high-energy, gravel-based Squamish River, sediments laid down on bar surfaces are composed of trough and planar crossbedded coarse sands. These sequences contrast incongruously with adjacent floodplain deposits which are composed in large part of vertically accreted fine sands atop coarse alluvial gravels. Using element analysis it is inferred that bar platform sediments are stripped away by chute channels, which are subsequently infilled with lower-energy deposits. From this, a model of floodplain growth based on selective preservation of bar platform sands and prefrential preservation of vertically accreted deposits is proposed. This mechanism of sediment replacement occurs independent of channel planform type.     

Holocene floodplain metamorphosis in the Midlands, United Kingdom

Many lowland rivers in the United Kingdom, including the Nene, Soar and Severn, have layered floodplains with a basal gravel of Pleistocene or Late glacial age and a structureless silty clay superficial unit burying the entire former floodplain. This burial is illustrated by the existence of variable, mixed and pedologically disturbed sediments and palaeo landsurfaces between the basal gravels and superficial silty clay. This paper presents a comparison of the pre- and post-late Holocene palaeo landsurfaces and palaeochannels using data from the Nene, Soar and Severn valleys. From this comparison it is argued that during the mid to late Holocene (ca 4500 yr BP to 2500 yr BP) floodplains and river channels underwent a metamorphosis. This is indicated by accelerated vertical accretion, a reduction in floodplain relative relief, changed floodplain soil conditions, a reduction in channel W/D ratios and a resultant increase in the silty clay proportion of channel perimeter sediments. There are indications that hydrological change preceded this metamorphosis but the primary cause was an increase in fine sediment supply during the later Holocene and a disequilibrium between channel bed and floodplain aggradation rates resulting in relative incision. This metamorphosis, which is explained in this paper by the proposed stable-bed aggrading-banks model (SBAB), is the key factor in the Holocene evolution of low-energy floodplain systems in the United Kingdom, upon which more subtle short-term fluctuations are superimposed.     

River stabilisation due to changing climate and vegetation during the late Quaternary in western Tasmania, Australia

The Stanley River in western Tasmania, Australia, contains sub-fossil rainforest logs within the channel and floodplain. Of the more than 85 radiocarbon dates obtained, all but 3 date from 17 ka to the present and permit an interpretation of fluvial and related environmental changes over this period. Particular attention is focused on the interactive relationship between the river and its riparian rainforest. Following the Last Glacial Maximum, the Stanley River was a laterally active gravel-load system reworking most of its valley floor in the upstream reaches. With ameliorating conditions at the end of the Pleistocene, climate became less seasonal and flow regimes less energetic. Huon pines already present in the catchment, re-asserted themselves in the form of dense tree cover along the river banks and floodplains with basal floodplain deposition shifting from gravels to coarse sands and granules. By about 3.5 ka, a further change in climate reduced stream discharges substantially. As a result the channel reduced in size, transported finer sediment, became laterally stable, and the floodplain accreted with overbank deposits of sand and silt. Huon pines falling into the channel formed obstructions of woody debris, some surviving for 2 ka. These have reduced stream power and boundary shear stress, further contributing to channel stability. Generational sequences of Huon pines on the river banks, some extending back 1–2 ka, are additional evidence of this stability. Since the Pleistocene, changing climate and the re-establishment of dense riparian rainforest appear to have stabilised the river channels and floodplains of western Tasmania.     

Cosmogenic nuclide budgeting of floodplain sediment transfer

Cosmogenic nuclides produced in quartz may either decay or accumulate while sediment is moved through a river basin. A change in nuclide concentration resulting from storage in a floodplain is potentially important in large drainage basins in which sediment is prone to repeated burial and remobilization as a river migrates through its floodplain. We have modeled depth- and time-dependent cosmogenic nuclide concentration changes for ¹⁰Be, ²⁶Al, and ¹⁴C during sediment storage and mixing in various active floodplain settings ranging from confined, shallow rivers with small floodplains to foreland-basin scale floodplains traversed by deep rivers. Floodplain storage time, estimated from channel migration rates, ranges from 0.4 kyr for the Beni River basin (Bolivia) to 7 kyr for the Amazon River basin, while floodplain storage depth, estimated from channel depth, ranges from 1 to 25 m.For all modeled active floodplain settings, the long-lived nuclides ¹⁰Be and ²⁶Al show neither significant increase in nuclide concentration from irradiation nor decrease from decay. We predict a hypothetical response time after which changes in ¹⁰Be or ²⁶Al concentrations become analytically resolvable. This interval ranges from 0.07 to 2 Myr and exceeds in all cases the typical residence time of sediment in a floodplain. Due to the much shorter half life of ¹⁴C, nuclide concentrations modeled for the in situ-produced variety of this nuclide are, however, sensitive to floodplain storage on residence times of < 20 kyr.The cosmogenic nuclide composition of old deposits in currently inactive floodplains that have been isolated for periods of millions of years from the river that once deposited them is predicted to either increase or decrease in ¹⁰Be and ²⁶Al concentration, depending on the depositional depth. These conditions can be evaluated using the ²⁶Al/¹⁰Be ratio that readily discloses the depth and duration of storage.We illustrate these models with examples from the Amazon basin. As predicted, modern bedload collected from an Amazon tributary, the Bolivian Beni River, shows no systematic change in nuclide concentration as sediment is moved through 500 km of floodplain by river meandering. In contrast, in the central Amazon floodplain currently untouched by the modern river system, low ²⁶Al/¹⁰Be ratios account for minimum burial depths of 5 to 10 m for a duration of > 5 Myr.The important result of this analysis is that in all likely cases of active floodplains, cosmogenic ¹⁰Be and ²⁶Al concentrations remain virtually unchanged over the interval sediment usually spends in the basin. Thus, spatially-averaged denudation rates of the sediment-producing area can be inferred throughout the entire basin, provided that nuclide production rates are scaled for the altitudes of the sediment-producing area only, because floodplain storage does not modify nuclide concentrations introduced from the sediment source area.     

The fluvial cycle at cold–warm–cold transitions in lowland regions: A refinement of theory

A previously established non-linear theory of river cyclicity as a response to climate change states that short phases of fluvial instability occur both at the transition from relatively warm (temperate) to cold (periglacial) and from relatively cold to warm periods. Such instability typically starts with vertical erosion, successively followed by sedimentary fill of the erosive scar. In spite of frequent confirmation of this theory by geomorphological reconstructions, a few problems arise. First, there are fewer incision phases than climatic transitions. Secondly, remnants of erosion at ‘cold–warm’ transitions are scarce, in contrast to obvious erosion relics at ‘warm–cold’ transitions. Furthermore, it appears that the incision style is strikingly different at both kinds of climatic transitions. Similarly, the long stable phases are also expressed in terms of different floodplain development in cold and warm periods. These arguments require a modification of the general non-linear theory.At the transition from relatively temperate to colder conditions, rivers transformed gradually from a regular, low-energy, single-channel course to a periodically high-energy, multi-channel type. The latter (braided) type is characterized by intense lateral movement, rather than by deep vertical erosion. This results in a well-expressed morphology of wide, extensive floodplains and terraces. In contrast, the linear and constrained, meandering channels incise with small width–depth ratio and build floodplains of limited lateral extent. Consequences are twofold: 1) the spatial limitation of the deeply incised, meandering valleys at the beginning of warm periods counts against their recognition; 2) relatively strong, lateral migration of the braided rivers removes most traces of previous (meandering) systems, which contrasts with the limited lateral activity of confined meandering channels. It means that in a ‘warm–cold–warm’ alternation generally only one phase of vertical erosion is preserved, the one that is caused by the high-energy, braided river at the start of the cold period.     

红山水库上游河床及支流调整分析

以西辽河红山水库修建后上游河道调整为例，说明了除来水来沙等的变化所引起的河流系统调整外，存在由于河流系统可调要素之间的相互联系，相互作用，在水库水位上升后造成的溯源淤积发展中，主、支流河道、上、下游河段冲淤相互影响，槽，滩淤积相互转换，地生态系统调整的影响下，产生的可调要素的复杂多向的调整过程。     

Late Holocene development of a floodplain along a small meandering stream, northern Québec, Canada

This paper describes the activity of a small meandering stream and the development of its floodplain during the last 4600 years (calendar years BP) in the northern boreal zone of Québec. Three trenches were excavated across the floodplain's full width and permitted the interpretation of morphosedimentary units in relation to modern analogs. Chronological controls within trenches was provided by the dendrochronological and radiocarbon dating of buried tree trunks. From 4600 to 2900 cal. BP and from 1000 to 120 cal. BP, the channel migrated and constructed its floodplain at very slow rates, mostly because of low flow velocities, vegetated streambanks and the cohesive texture of marine sediments reworked by the channel. Vertical accretion rates were extremely variable on the floodplain, with high rates proximal to the modern channel and low rates over distal (also older) portions of the floodplain. Following a major channel shift (meander cut-off or avulsion) around 2900 cal. BP, channel migration appears to have been constrained to a narrow zone adjacent to the modern channel. Within this constrained zone, the migrating channel has reworked its own sediments leading to a marked unconformity between 2900–1000 cal. BP. It is thought that underlying marine sediments protrusions, and perhaps the forested banks, protected older alluvial sediments from being eroded during the last three millenniums. Our study shows that small boreal floodplains may contain, in a very small area, abundant and diversified archives of their evolution.     

Geomorphology and dynamics of the Mfolozi River floodplain, KwaZulu-Natal, South Africa

The geomorphology and dynamics of the Mfolozi River floodplain and estuary, located in the subtropical region of northern KwaZulu-Natal, South Africa, were considered with respect to existing models of avulsion and alluvial stratigraphy. The Mfolozi River floodplain may be divided into regions based on longitudinal slope and dominant geomorphic processes. Confinement of the Mfolozi River above the floodplain has led to the development of an alluvial fan at the floodplain head, characterized by a relatively high sedimentation rate and avulsion frequency, at a gradient of 0.10%. The lower floodplain is controlled by sea level, with an average gradient of 0.05%. Between the two lies an extremely flat region with an average gradient of 0.02%, which may be controlled by faulting of the underlying bedrock.Avulsion occurrences on the Mfolozi floodplain are linked to the two main zones of aggradation, the alluvial fan at the floodplain head, and toward the river mouth in the lower floodplain. On the alluvial fan, normal flow conditions result in scour from local steepening. During infrequent, large flood events, the channel becomes overwhelmed with sediment and stream flow, and avulses. The resulting avulsion is regional, and affects the location of the channel from the floodplain head to the river mouth. Deposits resulting from such avulsions contribute significantly to the total volume of sediment stored in the floodplain, and tend to persist for long periods after the avulsion. Contrastingly, on the lower floodplain, reaching of the avulsion threshold is not necessarily linked to large flood events, but rather to long-term aggradation on the channel that decreases the existing channels gradient while increasing its elevation above the surrounding floodplain. Resultant avulsions tend to be local and do not contribute significantly to the overall volume of floodplain alluvium.     

Bank Erosion in Fifteen Tributaries in the Glaciated Upper Susquehanna Basin of New York and Pennsylvania

The proportional contributions of cultivated lands and stream banks as sources of fine sediment loads were quantified in 15 rural watersheds in the Glaciated Appalachian Plateau region of the Susquehanna River basin of New York and Pennsylvania. We utilized a relatively simple method of fingerprinting sediment sources by comparing the concentrations of the nuclear bomb-derived radionuclide ¹³⁷ Cs in fluvial sediment samples collected from channel margins with sediment from cultivated fields and stream banks. The proportion of fine sediment from bank erosion ranged from none to 100% in the study tributaries, with a median contribution of 53% across the 15 study streams. In one stream with no evidence of bank sediment, anomalously high ¹³⁷ Cs levels in the samples indicated that the sources were pasture or forest, probably scoured from marshy floodplains upstream of the sampling sites. In the 14 other streams, cultivated lands accounted for an average of 42% of the fine sediment. We discuss sources of eroded bank material and the processes driving stream bank erosion in this glaciated region, and examine the impact of historic mill-dam deposits on bank erosion.     

Flood pattern and weather determine Populus leaf litter breakdown and nitrogen dynamics on a cold desert floodplain

Patterns and processes involved in litter breakdown on desert river floodplains are not well understood. We used leafpacks containing Fremont cottonwood (Populus deltoides subsp. wislizenii) leaf litter to investigate the roles of weather and microclimate, flooding (immersion), and macroinvertebrates on litter organic matter (OM) and nitrogen (N) loss on a floodplain in a cool-temperate semi-arid environment (Yampa River, northwestern Colorado, USA). Total mass of N in fresh autumn litter fell by 20% over winter and spring, but in most cases there was no further N loss prior to termination of the study after 653 days exposure, including up to 20 days immersion during the spring flood pulse. Final OM mass was 10–40% of initial values. The pattern of OM and N losses suggested most N would be released outside the flood season, when retention within the floodplain would be likely. The exclusion of macroinvertebrates modestly reduced the rate of OM loss (by about 10%) but had no effect on N dynamics over nine months. Immersion in floodwater accelerated OM loss, but modest variation in litter quality did not affect the breakdown rate. These results are consistent with the concept that decomposition on desert floodplains progresses much as does litter processing in desert uplands, but with periodic bouts of processing typical of aquatic environments when litter is inundated by floodwaters. The strong dependence of litter breakdown rate on weather and floods means that climate change or river flow management can easily disrupt floodplain nutrient dynamics.     

Record of recent river channel instability, Cheakamus Valley, British Columbia

Rivers flowing from glacier-clad Quaternary volcanoes in southwestern British Columbia have high sediment loads and anabranching and braided planforms. Their floodplains aggrade in response to recurrent large landslides on the volcanoes and to advance of glaciers during periods of climate cooling. In this paper, we document channel instability and aggradation during the last 200 years in lower Cheakamus River valley. Cheakamus River derives much of its flow and nearly all of its sediment from the Mount Garibaldi massif, which includes a number of volcanic centres dominated by Mount Garibaldi volcano. Stratigraphic analysis and radiocarbon and dendrochronological dating of recent floodplain sediments at North Vancouver Outdoor School in Cheakamus Valley show that Cheakamus River aggraded its floodplain about 1–2 m and buried a valley-floor forest in the early or mid 1800s. The aggradation was probably caused by a large (ca. 15–25×10⁶ m³) landslide from the flank of Mount Garibaldi, 15 km north of our study site, in 1855 or 1856. Examination of historical aerial photographs dating back to 1947 indicates that channel instability triggered by this event persisted until the river was dyked in the late 1950s. Our observations are consistent with data from many other mountain areas that suggest rivers with large, but highly variable sediment loads may rapidly aggrade their floodplains following a large spike in sediment supply. Channel instability may persist for decades to centuries after the triggering event.     

Erosion and sediment yields as influenced by coupled eolian and fluvial processes: The Yellow River,China

Based on data from the middle Yellow River, a model of erosion and sediment yield is proposed to describe coupled eolian and fluvial processes in a transitional zone from arid to sub-humid climates, and to explain rapid erosion and a high sediment yield in the zone. In the study area, wind action predominates from March to June, which erodes weathered bedrock and transports eolian sand to gullies, river channels and floodplains. In the following summer, especially from July to September, rainstorm runoff in gullies and river channels transports large quantities of fine loessic material, in the form of hyperconcentrated flow. As a result, most of the previously stored eolian sand and material supplied by mass-wasting of loess can be transported to the major tributaries and the main stream of the Yellow River, resulting in the high specific sediment yield. There exists an optimal grain size composition which maximizes suspended sediment concentration in the study area, resulted from the combined wind–water processes.     

Railroads, roads and lateral disconnection in the river landscapes of the continental United States

Railroads and roads are ubiquitous features in the river corridors of the United States. However, their impact on hydrologic, geomorphic, and ecological processes in fluvial and riparian landscapes has not been systematically explored at regional or continental extents. This study documents the geographic distribution of roads and railroads in the alluvial floodplains of the continental United States and the regional variability of their potential impacts on lateral connectivity and resultant channel and floodplain structure and function. We use national scale data sets and GIS analysis to derive data on stream–transportation network interactions in two broad categories: (1) crossing impacts, such as bridges and culverts, and (2) impacts where transportation infrastructure acts as a longitudinal dam along the stream channel, causing lateral floodplain disconnection. Potential stream crossing impacts are greatest in regions with long histories of road and railroad development and relatively low relief, such as the Mid-Atlantic, New England, and the Lower Mississippi and Ohio Valleys. Potential lateral disconnections are more prevalent in rugged regions such as the Western U.S. and Appalachians where transportation routes follow river corridors along valley bottoms. Based on these results, we develop a conceptual model that suggests that the area of lateral disconnection due to transportation infrastructure should be most extensive in mid-sized alluvial valleys in relatively rugged settings. The result of this disconnection is the disruption of the long-term, cut-and-fill alluviation and of the shorter-term flood and flow pulse processes that create and maintain ecosystem function in river landscapes. The tremendous extent of transportation infrastructure in alluvial valleys documented in this study suggests a revision to H.B.N. Hynes' statement that the valley rules the stream. Instead, it appears that in modern landscapes of the U.S. the valley rules the transportation network — and the transportation network rules the stream.     

Coupling relationships: Hillslope–fluvial linkages in the Hodder catchment, NW England

Variations in the coupling of sediment transfer between different parts of a fluvial catchment, e.g., hillslope to axial stream, can hamper understanding but are an integral part of the geomorphological record. Depositional environments respond to a combination of land use, climate, storms (floods), and autogenic conditioning. The distribution of sediment in the upland landscapes of NW England is out of equilibrium with contemporary climate and geomorphological processes; more a function of peri- and paraglacial mobilisation of glacigenic deposits. Soil and vegetation development after deglaciation have interrupted any progression toward sediment exhaustion with sediment release controlled largely by extrinsic perturbation, with late Holocene anthropogenic activity, climate and extreme hydrological events the likely candidates. This paper presents a new radiocarbon-dated Holocene geomorphological succession for the River Hodder (NW England), alongside evaluating new palaeoecological and geoarchaeological data to discern the impacts of human activity. These data show a late Holocene expansion in human occupation and use of the landscape since the Iron Age (700–0 cal. B.C.), with more substantial changes in the character and intensity of upland land use in the last 1300 years. The geomorphological responses in the uplands were the onset of considerable and widespread hillslope erosion (gullying) and associated alluvial fan development. Interpretation of the regional radiocarbon chronology limits gullying to four, more extensive and aggressive phases after 500 cal. B.C. The downstream alluvial system has responded with considerable valley floor deposition and lateral channel migration that augmented sediment supply by remobilising the existing floodplain terraces and led to the aggradation of a series of inset alluvial terraces. The timing of these changes between states of aggradation and incision in alluvial reaches reflects the increased connectivity between the hillslope and alluvial systems. Aspects of both the regional climate and land use histories are conducive to increasing discharge and sediment flux, but the region wide lowering of erosion thresholds appears a key driver conditioning these sediment-rich conditions and producing a landscape that was more susceptible to erosion under lower magnitude flows.     

MONITORING GEOMORPHOLOGICAL DISTURBANCE AND RECOVERY IN COMMERCIALLY LOGGED TROPICAL FOREST, SABAH, EAST MALAYSIA, AND IMPLICATIONS FOR MANAGEMENT

Resource development in the form of cyclical commercial logging activities results in a short period of often severe land disturbance followed by a prolonged phase of recovery. The monitoring of catchment sediment yield gives some indication of the gross erosion processes within the catchment as a result of the disturbance and may also effectively measure any ameliorative processes. Studies in Ulu Segama, Sabah, East Malaysia, recorded significant increases in stream suspended sediment loads as a result of logging. Stream loads were derived from a combination of daily sample measurements and storm event sampling using automated liquid samplers. Measured loads were then compared to computed loads from sediment discharge rating sets derived from the actual samples. Application of these ratings in the disturbed catchment initially underestimated sediment loads; however, a progressive overestimation of loads occurred as the catchment vegetation recovered. When using sediment rating curves as a tool to measure catchment erosion rates in disturbed environments, considerable caution has to be used. Vegetation recovery is reflected in the rapid recovery of stream water quality, making it necessary to continually review the rating.     

Sediment fluxes and buffering in the post‐glacial Indus Basin

The Indus drainage has experienced major variations in climate since the Last Glacial Maximum (LGM) that have affected the volumes and compositions of the sediment reaching the ocean since that time. We here present a comprehensive first‐order source‐to‐sink budget spanning the time since the LGM. We show that buffering of sediment in the floodplain accounts for ca. 20–25% of the mass flux. Sedimentation rates have varied greatly and must have been on average three times the recent, predamming rates. Much of the sediment was released by incision of fluvial terraces constructed behind landslide dams within the mountains, and especially along the major river valleys. New bedrock erosion is estimated to supply around 45% of the sedimentation. Around 50% of deposited sediment lies under the southern floodplains, with 50% offshore in large shelf clinoforms. Provenance indicators show a change of erosional focus during the Early Holocene, but no change in the Mid–Late Holocene because of further reworking from the floodplains. While suspended loads travel rapidly from source‐to‐sink, zircon grains in the bedload show travel times of 7–14 kyr. The largest lag times are anticipated in the Indus submarine fan where sedimentation lags erosion by at least 10 kyr.     

Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches.In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels.The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.